The typical manufacturing process for memory hard disk media involves plating an aluminum disk substrate with a layer of nickel phosphorus and polishing the nickel alloy to obtain a smooth and flat surface. This polished nickel alloy's surface is suitable for the application of a hard disk's magnetic storage layer. The computer industry's requirements for increased storage capacity on hard disk drives has necessitated a substantial increase in areal density, i.e., data storage capacity per unit surface area, on the disk media. This demand for hard disks with higher areal densities has required several manufacturing improvements that include enhanced plating uniformity, reduced surface roughness after polishing and enhanced texturing characteristics. The polishing process is an important factor that influences many of these new requirements.
In addition to these new manufacturing requirements, significant improvements in surface inspection methodology have allowed disk manufacturers to inspect for smaller surface defects. This advanced inspection technology has led to optimization of polishing parameters such as polishing time, pressure and revolution rate of a polishing machine's upper and lower tables. Optimizing these parameters typically reduces surface defects that can occur during polishing. This technique however requires a high degree of expertise; and it is quite time consuming. Other advances have been made in the area of consumables: polishing pads, abrasive slurry, and cleaning materials.
Although these techniques all reduce surface defects, surfaces with various types of defects do result in uneven plating of the magnetic layer. For example, surface defects on the magnetic layer such as nodules that reduce the clearance between the magnetic head and the magnetic layer to less than 0.2 .mu.m may damage or even crush the magnetic head. Other defects such as scratches and pits may result in errors in reading or writing information on hard disks.
Manufacturers have experienced limited success with eliminating errors that arise from conventional aluminum oxide slurries. These high-tech slurries use aluminum oxide of various surface area (5 to 50 m.sup.2 /g) and size distribution (0.1 to 10 .mu.m) as the polishing agent. Unfortunately, these slurries' large particle sizes cause micro-scratches, micro-pits and nodules on nickel-plated substrates' surface. There are several possible causes for these defects including: 1) the grinding action of aluminum oxide on substrate surfaces introduces polish scratches; 2) the accumulation of unwanted wastes from the oxidized substrate and nickel debris mixed with the water to reduce the effectiveness of the injected polishing slurry; and 3) agglomeration of the abraded debris in the pores of the polishing pads scratches and pits the substrate's surface. In view of all these factors, it is difficult for disk manufacturers to achieve their required smooth surface, i.e., a roughness (peak-to-valley height) of less than 3 .ANG. or even 6 .ANG. with conventional alumina-based slurries.
Manufacturers have suggested that surface defects form primarily during the nickel plating process. In an attempt to correct these plating defects, most proposed solutions have increased the polishing rate to reduce or eliminate surface irregularities. For example, chemical additives such as chelating agents and oxidizers promote oxidation and accelerate the polishing rate of slurries. Others have used smaller or softer alumina-based abrasive particles to minimize polish scratches on substrate surfaces.
These attempts for eliminating surface irregularities have primarily focused on increasing the rate of chemical etching and mechanical abrasion of the nickel plated on the aluminum substrate, for example, by applying a high polishing rate. It is however necessary to increase the particle size of aluminum oxide in order to provide sufficient mechanical abrasion. Unfortunately, the use of large particles also tends to roughen the polished surfaces. Thus, it is often difficult, if not impossible, to achieve a high polishing rate and low roughness simultaneously. Other slurry manufacturers have used strong oxidizers or accelerators in slurries to increase the oxidizing rate of nickel plated on aluminum substrates. These chemically aggressive slurries may, however, cause pitting on the substrate surfaces and result in an undesirable buildup of slurry and nickel debris in the polishing pads. An excessive build up of unwanted waste in polishing pads leads to: defects on substrate surfaces; longer pad scraping between polishing cycles; and reduced pad life.
Wang et al., in U.S. Pat. No. 5,693,239 disclose the use of submicron alumina for chemical-mechanical polishing. Okajima et al., in U.S. Pat. No. 4,956,015 combine .alpha.-alumina with boemite for improved polishing removal rates. As far as known, these slurries lack the combination of removal rate and performance demanded by current disk manufacturers.
Furthermore, there have been other unsuccessful attempts to prevent the formation of surface defects by using smaller and softer conventional abrasives powders; these slurries however, often form pits and nodules due to their slow polishing rates. In addition, prior attempts to polish nickel-plated aluminum substrates include using a silica-only slurry. For example, PCT Pat. Pub. No. 98/23697 discloses polishing with slurries that contain 5 weight percent silica. Similarly, U.S. Pat. No. 5,733,819 to Kodama et al. discloses the use of fumed silica with malic acid. Finally, Kodama et al., in U.S. Pat. No. 5,575,837, disclose a silica gel with a relatively strong persulfate accelerator for increasing polishing removal rates. Although these fumed silica processes obtain a smooth surface, their polishing rate are too slow for many commercial applications--even when adding a large amount of oxidizers or accelerators to the slurry. Furthermore, these slurries can become too acidic for safe handling by operators on a daily basis.
It is an object of this invention to provide a polishing slurry for producing flat-smooth surfaces.
It is a further object of this invention to provide a polishing slurry for disk manufacturers to achieve a surface roughness of less than 6 .ANG..
It is a further object of this invention to produce a smooth surface with removal rates analogous to alumina-based slurries.